Image pickup apparatus are available wherein a still picture mode for picking up a still picture and a moving picture mode for picking up moving pictures are selectively changed over to record moving pictures onto a tape-like recording medium and record a still picture into a semiconductor recording medium. One of image pickup apparatus of the type mentioned is a video camera wherein a CCD (charge-coupled device) unit which uses more than 1,000,000 pixels in an effective screen region upon still picture image pickup as an image pickup element to recode moving pictures onto a tape-like magnetic recording medium and record a still picture into a removable semiconductor recording medium.
In the video camera described above, upon still picture image pickup, image information over the overall effective screen region is read out. However, upon moving picture image pickup, only image information within a screen region narrower than that upon still picture image pickup is read out effectively. The reason is such as follows. Since the number of frames and the number of fields per 1 second are determined by the prescription of a television system, it is necessary to read out all information from those pixels in the effective screen region for a moving picture within the thus determined period of time of one frame. In order to read out a number of pieces of pixel information as great as the number of pieces of pixel information for a still picture within the limited period of time, it is effective to raise the driving frequency for the CCD unit. However, if it is tried to read out pixel information from all of the effective pixels for each frame, then the frequency for driving the CCD unit becomes so high that the amplitude of the current waveform obtained from the CCD unit cannot be transmitted correctly. Therefore, the number of pixels to be read out for a moving picture is limited to an upper limit to the frequency with which the CCD unit can be driven and the period of time of one frame in a television system. Therefore, it cannot sometimes be avoided to set the effective screen size smaller than that upon still picture image pickup (to reduce the number of pixels to be read out effectively). Further, as the driving frequency for the CCD unit increases, the level of unnecessary radiations from the image pickup apparatus rises, and this has a bad influence on another communication equipment or electronic equipment.
Also a video camera is available wherein a region of a CCD unit from which pixel information is not read out upon moving picture image pickup as described above is used to successively move a cut-out position for a moving picture screen for every frame based on information of detection of a camera shake to perform camera-shake correction.
Accordingly, in such video cameras as described above, since the sizes of the effective screen regions for a moving picture and for a still picture are different from each other, when the video camera is changed over from a still picture image pickup state to a moving picture image pickup state, the angle of view is reduced at a wide angle end (the angle of view is shifted to the tele-side). This sometimes gives a sense of incongruity to the user.
For example, a video camera wherein the effective pixel number upon still picture image pickup is approximately 1,000,000 and approximately 690,000 pixels are read out in a period of time for one frame upon moving picture image pickup is known as an example. In another video camera wherein the effective pixel number upon still picture image pickup is approximately 1,400,000, the effective screen for a moving picture is divided into two left and right portions, and different drive circuits are used for the left and right portions to read out pixel information. Then, the thus read out left and right screens are joined together by an image processing circuit to form a screen for one frame. Thus, the image pickup element is divided into two left and right portions and the driving frequency for the left and right screens is lowered to achieve use of a greater number of pixels. In the example described, while the effective pixel number upon still picture image pickup is approximately 1,400,000, the effective pixel number upon moving picture image pickup is approximately 970,000. Thus, a result is achieved that the tele-shift ratio upon changeover from the still picture image pickup state to the moving picture image pickup state can be raised to approximately 1.2 times as substantially equal to that of the video camera described above which has approximately 1,000,000 pixels.
Also a further video camera has been realized wherein the effective pixel number upon still picture image pickup is approximately 1,920,000. However, the video camera adopts, as a countermeasure for lowering the driving frequency for an image pickup element, only a technique of dividing the effective screen into two left and right portions to read out pixel information similarly to the video camera described above which has approximately 1,400,000 pixels. Therefore, the tele-shift ratio upon changeover to moving picture image pickup rises to approximately 1.33 times, resulting in further increase of the sense of incongruity of the user.
It is to be noted that a zoom lens used in the video camera described above includes second to fourth lens sets having movable positions and arranged so as to have positive, negative, positive and positive refractive powers in order from the object side. One of such zoom lenses is disclosed in Japanese Patent Laid-Open No. Sho 62-178917 (hereinafter referred to as patent document 1). Another one of such zoom lenses is disclosed in Japanese Patent Laid-Open No. Hei 4-13110 (hereinafter referred to as patent document 2) which includes second to fourth lens sets having movable positions and arranged so as to have positive, negative, positive, positive and positive refractive powers in order from the object side. The zoom lenses disclosed in the patent documents and Japanese Patent Publication No. Sho 57-15369 (hereinafter referred to as patent document 3) have a tendency that, where the zoom ratio is set to approximately 10, if the angle of view at the wide angle end is set to an extremely wide angle with respect to that corresponding to f=40 mm where it is converted into an angle of view of a silver salt film camera of the 35 mm size (such conversion is hereinafter referred to as 35 mm size conversion), then the front lens diameter becomes great as much and this makes the overall size of the zoom lens great.
The problem that a sense of incongruity is given to a user because the angle of view at the wide angle end becomes narrow when the video camera described above is changed over from a still picture mode to a moving picture mode can be eliminated if the following countermeasure is taken. In particular, the zoom range of the zoom lens used in the video camera described above is expanded from the zoom ratio of approximately 10 times to the wide angle side to raise the zoom ratio to 12 times. Then, in a still picture mode, the zoom lens is used as a 10-time zoom lens cutting off a zoom range on the wide angle side, but in a moving picture mode, the lens is used as a 12-time zoom lens. However, if the zoom ratio is raised in vain only in order to prevent such a sense of incongruity to a user, then the overall size of the entire zoom lens is increased.
In conformity with the improvement in print quality of a printer in recent years, it is demanded to further improve the picture quality of a still picture of an image pickup apparatus, and in the field of digital still cameras designed principally for still picture image pickup, those digital still cameras wherein the effective pixel number of approximately 3,000,000 are prevailing. Further, also in the field of digital still cameras designed principally for still picture image pickup, those digital still cameras which additionally have a moving picture image pickup function and can record moving picture images which do not conform to a television system into a semiconductor recording medium similarly to a picked up still image are increasing. However, since the recording time for moving pictures is short and the recording system does not conform to a television system, the moving picture image pickup function is a mere additional function at all.
Thus, as a potential demand of consumers, it is demanded to use a single image pickup apparatus which has both of a function of recording a still picture of a high picture quality and another function of recording moving pictures in conformity with a television system for a long period of time and can record a still picture of a high picture quality into a semiconductor recording medium and record moving pictures onto a video tape having a long recording period of time through arbitrary selection.
However, where the effective pixel number of a still picture exceeds approximately 2,000,000, only if the screen for a moving picture is divided into two left and right portions as described above as a countermeasure for reducing the driving frequency for an image pickup element, a sufficient effect cannot be anticipated. Rather, it is estimated that the tele-shift ratio upon changeover from still picture recording to moving picture recording becomes higher than 1.33 times, resulting in further increase of the sense of incongruity which the user may have.
Ideally, a zoom lens is desirable which exhibits no change in angle of view even if the size of the effective screen region of the image pickup element changes as a result of changeover between image pickup modes for a still picture and moving pictures. However, since it is effective, upon moving picture image pickup, to correct a camera shake in order to improve the picture quality, a camera shake correction region within which the position at which the effective screen region of the image pickup element is to be cut out is varied in accordance with a camera shake need be covered widely with an effective image circle of the zoom lens. Therefore, it is permitted that the angle of view of the effective screen region for a moving picture becomes narrower than the angle of view of the effective image circle of the zoom lens by an amount by which the effective screen region of the image pickup element is cut with the camera shape correction region. Further, the camera shake correction region need not necessarily have a fixed screen area over the overall zoom region of the zoom lens, but may have a small area at the wide angle end if it is taken into consideration to fix the camera shake correction angle. Thus, the camera shake correction region need not necessarily occupy 20% as in the case of the video camera described hereinabove.
Therefore, it is a possible idea to employ a method wherein a new lens system is additionally provided for a zoom lens to shift the focal length range to moderate the change of the angle of view between upon moving picture image pickup and upon still picture image pickup in order to eliminate the sense of incongruity of the user upon changeover between the still picture image pickup mode and the moving picture image pickup mode.
It is to be noted that, as a technique for shifting the focal length range of a zoom lens, a method wherein a so-called tele-conversion lens having a negative refractive power is interposed between the zoom lens and an image pickup plane to shift the focal length in the overall power variation region to the longer side is known. Another method wherein a lens system called extender is inserted in a relay lens system of a zoom lens to shift the focal length to the longer side is also known. However, none of the method changes the dimension of the effective screen region of the image pickup element in response to shift of the focal length.
Incidentally, one of methods wherein a so-called tele-conversion lens having a negative refractive power is interposed between a zoom lens and an image pickup plane to shift the focal length in the overall power variation region to the longer side employs a configuration similar to that of the zoom lens disclosed in the patent document 1. Referring to FIGS. 38A and 38B, according to the method mentioned, a zoom lens “a” has a four-lens set configuration including first to fourth lens sets G1 to G4 of a positive-negative-positive-positive refractive power arrangement, and a negative lens system “b” is moved to and from a position on an optical axis between the fourth lens set G4 and an image pickup element IMG to shift the focal length range. In the zoom lens “a” described above, when the negative lens system “b” is positioned on the optical axis, the focal length over the overall power variation region is shifted to the longer side.
In the zoom lens “a” shown in FIG. 38B which uses the negative lens system “b” in order to shift the focal length range, the first lens set G1 and the third lens set G3 each having a fixed position with respect to the image plane during zooming are preferably held integrally in a fixed barrel together with the image pickup element IMG. In particular, where the first lens set G1 and the third lens set G3 which are fixed lens sets each having a fixed position and the image pickup element IMG are retained integrally in a lens barrel, it is easy to keep the distances between the lens sets and the distances from the lens sets to the image pickup element in good accuracy. Also it is easy to keep such error factors as inclinations of the lens sets with respect to the optical axis within respective necessary tolerances.
However, where the method as used by the zoom lens “a” wherein the negative lens system “b” is moved to and from the position on the optical axis is used, it is necessary to move the image pickup element IMG in a direction of the optical axis with respect to the first lens set G1 and the third lens set G3 which are fixed lens sets. Where the image pickup element IMG is supported for movement, a complicated mechanism is required for maintaining the accuracy in position of the image pickup element IMG in the direction of the optical axis and preventing inclination of the image pickup element IMG with respect to the optical axis. Consequently, there is a problem that the complicated mechanism is likely to make a significant factor of dispersion in performance of the zoom lens “a” upon mass production. Further, there is another problem that foreign articles such as dust are likely to be admitted to the proximity of the zoom lens “a” from the outside and are picked up as a shadow in a picked up image.
Further, according to the method wherein the focal length over an overall power variation region of a zoom lens is shifted to the longer side by means of a built-in extender, a wide air distance is provided in a relay lens system of the zoom lens in advance and the extender proximate to an afocal system like a telescope is inserted in the air distance. Accordingly, although the overall length of the zoom lens does not vary upon shifting of the focal length range, the overall length is liable to become long, and therefore, the method is disadvantageous in terms of miniaturization. Also it is a problem that, since the lens configuration of the extender to be inserted requires three to five lenses, the cost also becomes high and the space for retracting the extender from the position on the optical axis is required in the lens barrel, resulting in increase of the volume of the lens barrel.
Further, in such video cameras as descried above, in order to moderate such a defect as a color false signal or a moiré effect caused by discrete sampling of an image by the image pickup element, it is necessary to cut high frequency components of an image by means of an optical low pass filter. Crystal having double refraction such as quartz or lithium niobate crystal is normally used for an optical low pass filter. An example of a manner of effective use of an optical low pass filter is disclosed in the patent document 3 by the assignee of the present patent application, and the thickness of a double refraction plate is determined in accordance with the pixel pitch of the image pickup element.
Thus, it is a possible idea as a further reduction countermeasure for the driving frequency for an image pickup element to extract information obtained by mixing information extracted from each plurality of adjacent pixels from among pixels of an image pickup element as information of one pixel to effectively form an image having a rougher pixel pitch. For example, information of four adjacent pixels may be mixed so that it is equivalently regarded as information of one pixel to make the pixel pitch in the moving picture mode rougher thereby to reduce the driving frequency for the image pickup apparatus.
However, even if the driving frequency for an image pickup element in the moving picture mode can be reduced in this manner, the following optical problem appears. In particular, the frequency characteristic of such an optical low pass filter as disclosed in the patent document 3 should be determined in accordance with the pixel pitch in the still picture mode. Thus, if it is determined in accordance with the pixel pitch used when information of a plurality of pixels is mixed in the moving picture mode, then the MTF (Modulation Transfer Function) of the still picture lowers. As a result, a high picture quality which makes the most of, for example, 3,000,000 or more effective pixels cannot be obtained. However, if a moving picture wherein information of a plurality of pixels is mixed to produce information of one pixel while the low pass characteristic conforming to the pixel pitch of the still picture mode is maintained, then a defect such as a color false signal or a moiré effect which is caused by discrete sampling of image information appears remarkably. This is not preferable to the picture quality of moving pictures.
Taking the foregoing problems described above, it is a subject of the present invention that, in an image pickup apparatus having a still picture image pickup mode and a moving picture image pickup mode, and a zoom lens for use with the image pickup apparatus, even if the dimension of an effective screen region of an image pickup element changes as a result of changeover of the image pickup mode between a still picture image pickup mode and a moving picture image pickup mode, the angle of view changes but less conspicuously and good aberration correction can be achieved both in the still picture image pickup mode and in the moving picture image pickup mode.
It is another subject of the present invention to provide, in an image pickup apparatus wherein an effective pixel pitch is changed over between a still picture mode and a moving picture mode, an appropriate optical low pass effect in each image pickup mode by changing over the characteristic of an optical low pass filter in accordance with changeover of the pixel pitch and to make it possible, in an image pickup apparatus wherein the pixel pitch and the effective screen size are changed over between the still picture mode and the moving picture mode, to simultaneously achieve changeover of the characteristic of the optical low pass filter and moderation of the change of the angle of view at the wide angle end of a zoom lens.